Their ongoing investigations into theft at Shell's Pulau Bukom site, just south of the country's main island, have so far entangled several international companies in incidents going back nearly six months.

Police have so far charged 14 men, arrested 20 and seized at least one oil tanker and millions of dollars in cash.

The latest charges were filed on Thursday. Here's what we know so far.

WHO'S INVOLVED?

Among the 14 men charged, there are 11 Singaporeans and three Vietnamese.

Eight of the Singaporeans were employees of Shell Eastern Petroleum (Pte) Ltd, the Singapore subsidiary of Royal Dutch Shell Plc, while two others worked for Sentek Marine &amp; Trading Pte Ltd, one of Singapore's biggest marine fuel suppliers.

According to an internal memo to all employees from Sentek Marine's Managing Director Pai Keng Pheng on January 13, Sentek has terminated the employment of the two, Marketing and Operations Manager Benny Ng Hock Teck and Cargo Officer Alan Tan Cheng Chuan.

The investigation found that one of two vessels used to transport the stolen oil products was Sentek 26, which is managed by Sentek Marine &amp; Trading.

Sentek was not charged or named in court documents.

The other Singaporean charged worked for Intertek, a British-listed company specializing in quality and quantity assurance, including for fuel products.

The Vietnamese nationals were charged with receiving stolen property aboard two other ships - Prime South and MT Gaea - both of which are Panama flagged.

Authorities have arrested 20 people in connection with the case, sources close to the matter said.

WHAT WAS STOLEN AND WHEN?

The court documents list thefts on several dates going back to July 2017, many of which happened during the refinery's normal working hours and involved millions of dollars' worth of gasoil.

Below is a list of the dates of the incidents and the amount and value of gasoil reportedly stolen. In some cases, currency conversions have been done by Reuters.

* July. 24, 765 metric tonnes valued at S$252,558

* Oct. 2, 1,131 metric tonnes valued at $579,445 (S$765,273)

* Oct. 23, 1,276 metric tonnes at $653,733 (S$863,385)

* Nov. 11, 300 metric tonnes (S$165,000)

* Nov. 14, 846 metric tonnes (S$465,520)

* Nov. 21, 2,322 metric tonnes (S$1,277,430)

* Dec. 1, 1,133 metric tonnes at $639,726 (S$847,317)

* Dec. 31, 1,076 metric tonnes (S$592,295)

* Dec. 31, 1,349 metric tonnes (S$736,445)

* Jan. 5, 802 metric tonnes (S$437,816)

* Jan. 7, 1,260 metric tonnes (S$687,960)

WHERE WAS THE OIL HEADED?

There have been three ships named in court documents as having received the stolen oil - Prime South, Sentek 26 and MT Gaea.

Over the last three months, those vessels have travelled between ports in Singapore, Vietnam, Thailand and Indonesia, shipping data from Thomson Reuters Eikon show.

WHAT'S NEXT?

Singapore police say investigations are still ongoing. Six of 17 men initially arrested have yet to face charges.

Some of those charged so far are due back in court on Jan. 22 and Jan. 25, and may face additional charges.